RFR: JDK-8312418: Add Elements.getEnumConstantBody

[Joe Darcy]

Trivial implementation to get feedback on the proposed API.

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Commit messages:
 - Appease jcheck.
 - Merge in patch from Jan.
 - Merge branch 'master' into JDK-8312418
 - Merge branch 'master' into JDK-8312418
 - Correct test predicate.
 - Augment test.
 - appease jcheck
 - Add test; check-point implementation.
 - Merge branch 'master' into JDK-8312418
 - Merge branch 'master' into JDK-8312418
 - ... and 2 more: https://git.openjdk.org/jdk/compare/2eae13c6...3684d30a

Changes: https://git.openjdk.org/jdk/pull/14939/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14939&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312418
  Stats: 316 lines in 4 files changed: 316 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14939.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14939/head:pull/14939

PR: https://git.openjdk.org/jdk/pull/14939